Optical holographic data storage provides a high density fixed data base. One such system involves the storage and retrieval of data from a Fourier transform transmission hologram. Such a system uses a lens arrangement for data retrieval and requires considerable space. The use of a transmission hologram in a Fourier transform data storage system precludes the use of a double-sided holographic memory plate.
Another conventional holographic data storage system increases storage capacity by recording multiple holograms at different polarizations. In such a system, the retrieval or playback is at different polarization angles. Such a system is typically complex and bulky.
Another prior optical data storage system is commonly called the "optical disc". In such a system, the data is stored in binary form as a series of small holes. The presence of a hole is equivalent to a 1 state, and the absence of a hole is equivalent to 0 state. The hole is very small, in the order of 1 to 2 micrometers. During the retrieval process, the disc is rotated at high speed. The distance between the disc and detecting head is very small, typically 2 to 10 micrometers. The disc can store a large amount of data, but accurate playback is very susceptible to vibration and movement. With high density information, the optical disc system is not usable in a vibrating environment such as a vehicle.
Accordingly, there is a need for high-density data storage having physical relationships such that the structures are compact and simple so that they can be built to resist vibrational disturbance experienced in moving equipment such as aircraft. Data storage in a series of reflective holographic elements is a system which can meet the high density, small size, short data access time requirements and severe environmental conditions which can occur in aircraft, space and other uses.